This is because it stops them from being able to run away, according to Stanley Coren from University of British Columbia in Canada who conducted the study. Coren analysed images of dogs being hugged by adults and children he found on the internet, looking for signs of stress, 'The Telegraph' reported.

When a dog is especially anxious it bares its teeth or may bite, but there are also 'subtler indicators' that it may be uncomfortable, Coren said.

There are different signs of stress and anxiety - the dog folding its ears down, half-moon eyes or turning its head away to avoid eye contact, closing its eyes, or licking its lips.

Yawning or raising one paw is another warning sign, said Coren. He found that the dogs had at least one sign of discomfort, stress, or anxiety in 81.6 percent of the photographs. Only 7.6 percent of the photographs showed comfortable dogs while the remaining 10.8 per cent were either neutral or ambiguous.

"Dogs are technically cursorial animals, which is a term that indicates that they are designed for swift running," said Coren.

"That implies that in times of stress or threat the first line of defence that a dog uses is not his teeth, but rather his ability to run away," Coren said.

"Behaviourists believe that depriving a dog of that course of action by immobilizing him with a hug can increase his stress level and, if the dog's anxiety becomes significantly intense, he may bite," he added. According to him, it is better for owners to express their fondness for the dog with a pat, a kind word, and maybe a treat.